Modern image displays are based on the concept of pixels. In the prior art a pixel represented the smallest element of an image which could be independently adjusted. A composite image was comprised of the sum of all of the individual pixels. Each pixel of an image must be generated by a pixel element: a device which causes a pixel to be displayed. Since a given image may be comprised of millions of pixels, millions of pixel elements may be required to display an image at its full resolution.
In the prior art there were two basic types of pixels: binary, which means that the intensity of a pixel is either fully OFF or fully ON, and gray-valued, which means that the intensity level of a pixel uniformly varies over a range of values, either continuously or in steps, from fully OFF to fully ON.
With either type of prior art pixel it is relatively easy to display an image. The pixel elements are simply set at a given intensity. However, displaying edges, particularly with high positional accuracy, is more difficult. An edge is a part of an image where the intensity makes a spatially rapid transition from one level to a contrasting level. Positional accuracy means the accuracy in placing an edge relative to other features in the displayed image. A fundamental problem with displaying an edge using prior art pixel elements is that each pixel element has a spatially uniform intensity: the intensity of a pixel element was spatially constant. Therefore, when an edge occurred within a pixel element, the position of that edge could only be approximated in discrete steps corresponding to the pixel element boundaries. However, this creates a problem. Because human vision is capable of detecting relative edge offsets at least ten times smaller than the smallest resolved periodic features, this effect being called edge acuity, the eye is sensitive to the discrete steps rendered by pixel element boundaries. Thus, very high positional accuracy is required to produce a high quality visual image.
One method of improving positional accuracy is to increase the pixel element density of a display. However, when the pixel element density increases beyond certain levels, the fabrication of the individual pixel elements and the storage and processing of the required image data become difficult. Increasing the pixel element density is wasteful in that it uses a high pixel element density simply to improve edge acuity.
Therefore, new types of display pixel elements which are capable of displaying subpixel image information would be advantageous.